Polybenzimidazole carbon fiber and method for manufacturing same

ABSTRACT

The present application provides polybenzimidazole carbon fiber that does not require infusibilization treatment and method for producing same.

TECHNICAL FIELD

The present invention relates to a polybenzimidazole carbon fiber madefrom a fiber material that is a precursor fiber includingpolybenzimidazole; and a method for producing the same.

BACKGROUND ART

Carbon fibers have been used in a wide variety of applications fromaircraft to building materials. If their productivity is improved andtheir cost is lowered more and more, they can be materials in place ofstainless steel plates also in automobile body and the like. At present,carbon fibers are mainly produced using polyacrylonitrile (PAN) fibersand pitch fibers as fiber raw materials (fiber precursor fibers).

These precursor fibers, however, require a pre-treatment called aninfusibilization treatment prior to carbonization, and this treatment isa major obstacle to reduction in cost and energy required for theirproduction, and to increase in productivity.

Specifically, because PAN fibers and pitch fibers are fused in thecourse of a carbonization treatment (a high-temperature thermaltreatment of 1,000° C. or more) and cannot maintain their fiber shapes,they are changed to infusible, flame-resistant fibers by an airoxidization treatment called an infusibilization treatment and then aresubjected to carbonization, to thereby obtain carbon fibers. In thisinfusibilization treatment, it is necessary to uniformly controloxidation reaction and also strictly manage temperature conditions forsuppressing thermal runaway due to exothermic reaction, and moreover itstreatment time is long (about 30 minutes to about 1 hour).

Therefore, the present inventors have studied various precursor fibersthat do not require the infusibilization treatment, and have presentedresearch reports of PBI carbon fibers obtained from polybenzimidazole(hereinafter referred to as “PBI”) fiber serving as precursor fibers(see NPL 1). The PBI fibers can be carbonized while maintaining theirfiber shape without performing the infusibilization treatment.

In existing reports, it is known that PBI fibers are spun and carbonizedto thereby obtain carbon fibers having an elastic modulus of 80 GPa anda strength of 670 MPa (see PTL 1). Moreover, it is known that carbonfibers having a diameter of more than 100 μm can be produced by treatingPBI fibers being basic with an acid solvent to thereby form salts.Moreover, it is believed that the above-described PBI fibers have anelastic modulus of 100 GPa and a strength of 420 MPa (see PTL 2).

However, the PBI carbon fibers obtained by carbonizing the PBI fibershave low elastic modulus and low strength, which is problematic.Therefore, the PBI carbon fibers are required to be improved in bothelastic modulus and strength for practical applications.

On the other hand, known is a method for removing, from fibers,polyphosphoric acid used in production of polymers by contacting the PBIfibers with a neutralization solution as a method for improving the PBIfibers serving as a precursor fiber in strength (see, for example, PTL3).

However, carbon fibers obtained from the above-described PBI fibersserving as precursor fibers have not been known. Moreover, the PBIcarbon fibers having sufficient elastic modulus and strength forpractical applications have not been found yet. That is, elastic modulusand strength of a precursor fiber do not always correspond to elasticmodulus and strength of a carbon fiber obtained by carbonizing thisprecursor fiber. Moreover, whether the carbon fiber can achieve intendedelastic modulus and strength is unknown. Therefore, there has been ademand that the PBI carbon fibers having sufficient elastic modulus andstrength for practical applications are newly developed.

CITATION LIST Patent Literature

-   PTL 1: U.S. Pat. No. 3,528,774-   PTL 2: U.S. Pat. No. 3,903,248-   PTL 3: Japanese Patent Application Laid-Open (JP-A) No. 2008-507637

Non-Patent Literature

-   NPL 1: Proceedings of the 39th Annual Meeting of The Carbon Society    of Japan, 3B02, 3B03 (2012)

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the above problems in the existingtechnique and achieve the following object. That is, an object of thepresent invention is to provide: a PBI carbon fiber that can beefficiently produced without an infusibilization treatment and isexcellent in elastic modulus and strength; and a method for producingthe PBI carbon fiber.

Solution to Problem

Means for solving the above problems are as follows.

<1> A polybenzimidazole carbon fiber including:

a structure obtained by turning a precursor fiber includingpolybenzimidazole into a carbon fiber under application of heat,

wherein the polybenzimidazole includes a structure represented byGeneral Formula (1) or General Formula (2) below as a structural unit,and

wherein the polybenzimidazole carbon fiber has an elastic modulus intension of 100 GPa or more and a tensile strength of 0.8 GPa or more:

where in the General Formulas (1) and (2), R¹ and R³ each represent atrivalent or tetravalent group of one selected from the group consistingof aryl groups and unsaturated heterocyclic groups that are expressed byany one of Structural Formulas (1) to (10) below, and R² represents abivalent group of one selected from the group consisting of aryl groupsand unsaturated heterocyclic groups that are expressed by any one of theStructural Formulas (1) to (10), alkenylene groups including from 2 to 4carbon atoms, an oxygen atom, a sulfur atom, and a sulphonyl group:

<2> The polybenzimidazole carbon fiber according to <1>, wherein thepolybenzimidazole carbon fiber is a continuous fiber having a fiberdiameter of 8 μm or more.

<3> A method for producing a polybenzimidazole carbon fiber, the methodincluding:

spinning, in an acid solution, a polymer including polybenzimidazoleincluding a structure represented by General Formula (1) or GeneralFormula (2) below as a structural unit, to thereby obtain a firstprecursor fiber of the polymer;

contacting the first precursor fiber with a basic solution, andneutralizing the acid solution remaining in the first precursor fiber tobe removed, to thereby obtain a second precursor fiber; and

heating the second precursor fiber at a temperature of from 1,000° C. to1,600° C. under an inert gas, to thereby turn the second precursor fiberinto a carbon fiber:

where in the General Formulas (1) and (2), R¹ and R³ each represent atrivalent or tetravalent group of one selected from the group consistingof aryl groups and unsaturated heterocyclic groups that are expressed byany one of Structural Formulas (1) to (10) below, and R² represents abivalent group of one selected from the group consisting of aryl groupsand unsaturated heterocyclic groups that are expressed by any one of theStructural Formulas (1) to (10), alkenylene groups including from 2 to 4carbon atoms, an oxygen atom, a sulfur atom, and a sulphonyl group:

<4> The method for producing a polybenzimidazole carbon fiber accordingto <3>, wherein the acid solution is polyphosphoric acid and the basicsolution is an ethanol solution of triethylamine.

<5> The method for producing a polybenzimidazole carbon fiber accordingto <4>, wherein the contacting is allowing the first precursor fiber topass through a bath of the ethanol solution of triethylamine for from 5seconds to 30 seconds to neutralize the polyphosphoric acid remaining inthe first precursor fiber to be removed.

<6> The method for producing a polybenzimidazole carbon fiber accordingto <3>,

wherein the spinning further includes: coagulating, in a coagulationbath, a reaction solution of the polymer obtained through polymerizationin a first acid solution, to thereby obtain a first coagulated matter ofthe polymer; contacting the first coagulated matter with a first basicsolution to neutralize the first acid solution remaining in the firstcoagulated matter to be removed, to thereby obtain a second coagulatedmatter; and dissolves the second coagulated matter in a second acidsolution to prepare a raw liquid for spinning, and spins the raw liquidfor spinning, to thereby obtain a first precursor fiber of the polymer,and

wherein the contacting is contacting the first precursor fiber with asecond basic solution, and neutralizing the second acid solutionremaining in the first precursor fiber to be removed, to thereby obtaina second precursor fiber.

<7> The method for producing a polybenzimidazole carbon fiber accordingto <6>, wherein the first acid solution is polyphosphoric acid, thefirst basic solution is an aqueous sodium hydrogen carbonate solution,the second acid solution is methanesulfonic acid, and the second basicsolution is an ethanol solution of triethylamine.

<8> The method for producing a polybenzimidazole carbon fiber accordingto <7>, wherein the contacting is allowing the first precursor fiber topass through a bath of the ethanol solution of triethylamine for from 5seconds to 30 seconds, and neutralizing the methanesulfonic acidremaining in the first precursor fiber to be removed.

<9> The method for producing a polybenzimidazole carbon fiber accordingto any one of <3> to <8>, wherein a heating temperature of the heatingis a temperature of from 1,200° C. to 1,400° C.

Advantageous Effects of Invention

According to the present invention, it is possible to solve the aboveproblems in the existing technique and to provide: a PBI carbon fiberthat can be efficiently produced without an infusibilization treatmentand is excellent in elastic modulus and strength; and a method forproducing the PBI carbon fiber.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an image presenting cross sections of PBI carbon fibersaccording to Example 3 obtained through an electron microscope.

FIG. 1B is an image presenting cross sections of PBI carbon fibersaccording to Example 10 obtained through an electron microscope.

FIG. 1C is an image presenting cross sections of PBI carbon fibersaccording to Comparative Example 1 obtained through an electronmicroscope.

FIG. 1D is an image presenting cross sections of PBI carbon fibersaccording to Comparative Example 1 obtained through an electronmicroscope.

FIG. 2A is a graph presenting measurement results of elastic modulus intension.

FIG. 2B is a graph presenting measurement results of tensile strength.

FIG. 3 is an explanatory view presenting presumption conditions ofreachable strength.

FIG. 4A is an image presenting cross sections of PBI carbon fibersaccording to Example 15 obtained through an electron microscope.

FIG. 4B is an image presenting cross sections of PBI carbon fibersaccording to Example 16 obtained through an electron microscope.

FIG. 5 is a graph presenting measurement results of density.

FIG. 6A is a schematic view presenting plane interval c/2 of carbonnetwork planes and stack thickness L_(c) of carbon network planes in agraphite crystal.

FIG. 6B is a schematic view presenting an optical system in measuringwide angle X-ray diffraction profile.

FIG. 7 is a graph presenting a relationship between plane interval c/2and stack thickness Lc of carbon network planes.

FIG. 8 is a graph presenting measurement results of cross-sectionalareas of microvoids.

FIG. 9 is a graph presenting measurement results of volume percentagesof microvoids.

DESCRIPTION OF EMBODIMENTS

(PBI Carbon Fiber)

A polybenzimidazole (PBI) carbon fiber of the present invention includesa structure obtained by turning a precursor fiber including PBI into acarbon fiber under application of heat. The PBI includes a structurerepresented by the following General Formula (1) or General Formula (2)as a structural unit. The PBI carbon fiber has an elastic modulus intension of 100 GPa or more and a tensile strength of 0.8 GPa or more.

In the General Formulas (1) and (2), R¹ and R³ each represent atrivalent or tetravalent group of one selected from the group consistingof aryl groups and unsaturated heterocyclic groups that are expressed byany one of the following Structural Formulas (1) to (10), and R²represents a bivalent group of one selected from the group consisting ofaryl groups and unsaturated heterocyclic groups that are expressed byany one of the Structural Formulas (1) to (10), alkenylene groupsincluding from 2 to 4 carbon atoms, an oxygen atom, a sulfur atom, and asulphonyl group.

Examples of the alkenylene group include a vinylene group.

The precursor fiber including the above PBI (PBI precursor fiber) can becarbonized while maintaining its fiber shape even without aninfusibilization treatment. Therefore, the carbon fibers can beefficiently produced compared to carbon fibers obtained from precursorfibers such as PAN fibers or pitch fibers, which require theinfusibilization treatment.

In addition, the PBI precursor fiber can be carbonized with highcarbonization yield. Therefore, it is possible to suppress distortion ofstructures due to pyrolysis gas generated and released duringcarbonization, and/or generation of voids (pores) (including foaming)which would reduce the mechanical strength of carbon fibers. Moreover,partly because the carbonization yield is high; i.e., the amount of gasand/or tar released by pyrolysis during carbonization is small, even inthe case where carbonization is performed under rapid heating, it ispossible to avoid instant generation of a large amount of decompositiongas, which makes it possible to perform carbonization treatment veryrapidly. Thereby, it is possible to carbonize thick fibers having largevolumes relative to their outer surfaces so that gas does not easilyescape during carbonization.

The PBI carbon fiber has an elastic modulus in tension of 100 GPa ormore and a tensile strength of 0.8 GPa or more; i.e., it is excellent inboth elastic modulus and strength.

The reason why the PBI carbon fiber can achieve the above-describedelastic modulus and strength is because an acid solution in the PBIprecursor fiber is neutralized by a basic solution to be removed in thebelow-described production method. The invention of the PBI carbon fiberis based on the finding that the precursor fiber obtained through theabove-described neutralization for removal can be turned into a carbonfiber while maintaining a fiber structure of the precursor fiber.

Here, the elastic modulus in tension and the tensile strength can bemeasured by a single fiber tensile test according to the JIS7606 method.

As described above, the PBI carbon fiber can maintain high elasticmodulus and high strength even if a thick fiber is carbonized to have alarger diameter. Commercially available products of carbon fibers (e.g.,PAN carbon fibers) generally have a fiber diameter of about 7 μm.However, the PBI carbon fiber can maintain high elastic modulus and highstrength not only when the fiber diameter is a small diameter of from 2μm to 8 μm (exclusive) but also when the fiber diameter is 8 μm or more,and is further increased to a large thickness of 16 μm or more. Here,the upper limit of the fiber diameter is about 30 μm.

Moreover, the PBI carbon fiber can be a continuous fiber (filament).

The above-described PBI carbon fiber according to the present inventioncan be produced by a method for producing the PBI carbon fiber accordingto present invention, which will be described hereinafter.

(Method for Producing PBI Carbon Fiber)

The method for producing the PBI carbon fiber includes a step ofobtaining a first precursor fiber, a step of obtaining a secondprecursor fiber, and a step of producing a carbon fiber.

<Step of Obtaining First Precursor Fiber>

The step of obtaining a first precursor fiber is a step of spinning, inan acid solution, a polybenzimidazole-including polymer having astructure expressed by the General Formula (1) or (2) as a structuralunit, to thereby obtain a first precursor fiber of the polymer.

In the General Formulas (1) and (2), R¹ and R³ each represent atrivalent or tetravalent group of one selected from the group consistingof aryl groups and unsaturated heterocyclic groups that are expressed byany one of Structural Formulas (1) to (10) below, and R² represents abivalent group of one selected from the group consisting of aryl groupsand unsaturated heterocyclic groups that are expressed by any one of theStructural Formulas (1) to (10), alkenylene groups including from 2 to 4carbon atoms, an oxygen atom, a sulfur atom, and a sulphonyl group.

Examples of the alkenylene group include a vinylene group.

The PBI may be a commercially available product or may be synthesized.

When the PBI is synthesized, it can be obtained by allowing, in the acidsolution, terephthalic acid (available from, for example, Wako PureChemical Industries, Ltd.) and 4,4′-bephenyl-1,1′,2,2′-tetramine(available from, for example, Aldrich) as starting materials to proceedto polycondensation reaction.

The polymer may be the PBI itself. Alternatively, the polymer may be acopolymer formed of a structural unit of the PBI and another structuralunit, or a polymer blend material obtained by combining the PBI withanother polymer so long as the effects of the present invention are notdeteriorated.

The precursor fiber may be a fiber material obtained from the polymeritself. However, the precursor fiber may be a fiber material obtained byadding any substituent to a terminal of the polymer so long as theeffects of the present invention are not deteriorated.

Examples of the any substituent include an ester group, an amide group,an imide group, a hydroxyl group, and a nitro group.

Methods of the spinning can be roughly divided into the following twomethods: a first method and a second method. The first method can beperformed by directly spinning, as a raw liquid for spinning, a reactionsolution obtained by allowing the polymer to proceed to polycondensationreaction in the acid solution. The second method can be performed in thefollowing manner. Specifically, the acid solution constituting thereaction solution is regarded as a first acid solution. A coagulatedmatter of the polymer is first obtained from the reaction solution, andthen the coagulated matter is dissolved in a second acid solution forspinning, to thereby obtain a reaction solution as a raw liquid forspinning. Then, the raw liquid for spinning is spun.

The acid solution used for the first method is not particularly limitedso long as it can dissolve the starting materials and the polymer to beproduced and can serve as a catalyst that promotes polymerization.Specific examples of the acid solution include polyphosphoric acid,polyphosphate ester, diphenyl cresyl phosphate, and methanesulfonic acidin which diphenyl cresyl phosphate or diphosphorus pentaoxide isdissolved. Among them, the polyphosphoric acid is preferable in terms ofcontrolling the polymerization reaction.

When the spinning is performed by the second method, the step ofobtaining a first precursor fiber includes a step of obtaining a firstcoagulated matter and a step of obtaining a second coagulated matter. Inaddition, the step of obtaining a first precursor fiber is a step ofobtaining a first precursor fiber of the polymer by spinning a rawliquid for spinning, the raw liquid for spinning being prepared bydissolving, in a second acid solution, the second coagulated matterobtained in the step of obtaining a second coagulated matter.

—Step of Obtaining First Coagulated Matter—

The step of obtaining a first coagulated matter is a step ofcoagulating, in the first acid solution, the reaction solution of thepolymer obtained through polymerization in a coagulation bath, tothereby obtain a first coagulated matter of the polymer.

The first acid solution can be the same one as the acid solution used inthe first method.

A coagulation liquid in the coagulation bath is not particularly limitedso long as the polymer can be coagulated. Examples of the coagulationliquid include water, alcohol, methanesulfonic acid, polyphosphoricacid, and dilute sulfuric acid. Among them, the water is preferable.

—Step of Obtaining Second Coagulated Matter—

The step of obtaining a second coagulated matter is a step of contactingthe first coagulated matter with a first basic solution, andneutralizing the first acid solution remaining in the first coagulatedmatter to be removed, to thereby obtain a second coagulated matter.

The first basic solution is not particularly limited so long as itneutralizes the first acid solution. Examples of the first basicsolution include an aqueous sodium hydrogen carbonate solution, anaqueous sodium hydroxide solution, potassium hydroxide, and an ethanolsolution of triethylamine. Among them, the aqueous sodium hydrogencarbonate solution is preferable because reduction in a degree ofpolymerization can be prevented.

Here, the coagulated matter may be washed with water or alcohol beforeor after washed with the first basic solution.

When the spinning is performed by the second method as described above,the second coagulated matter that has been washed is dissolved in thesecond acid solution to prepare the raw liquid for spinning.

The second acid solution is not particularly limited so long as thesecond coagulated matter can be dissolved. Examples of the second acidsolution include methanesulfonic acid, polyphosphoric acid, andconcentrated sulfuric acid. Among them, the methanesulfonic acid ispreferable because it can impart viscosity suitable for the spinning tothe raw liquid for spinning.

The spinning method in the first method and the second method is notparticularly limited and may be appropriately selected depending on theintended purpose. Examples of the spinning method include known wet-typespinning methods and known dry-type spinning methods.

Note that, the first precursor fiber and a second precursor fiber thatwill be described hereinafter may be subjected to drawingtreatment•thermal treatment if necessary. Regarding the drawingtreatment, spun yarn may be directly drawn in a coagulation bath, orwound yarn may be washed with water and then drawn in the bath. Thedrawing treatment and the thermal treatment may be performed at the sametime. Regarding the thermal treatment, an atmosphere is not particularlylimited, but the thermal treatment is preferably performed in air or ina nitrogen atmosphere. A temperature and time of the thermal treatmentmay be appropriately selected, but the temperature of the thermaltreatment is preferably from 200° C. to 600° C. Moreover, a draw ratiois preferably from about 1.2 times to about 10 times.

As described above, the first precursor fiber can be obtained.

<Step of Obtaining Second Precursor Fiber>

The step of obtaining a second precursor fiber is a step of contactingthe first precursor fiber with a basic solution, and neutralizing theacid solution remaining in the first precursor fiber to be removed, tothereby obtain a second precursor fiber.

When the step of obtaining a first precursor fiber is performed by thefirst method, the basic solution used in the step of obtaining a secondprecursor fiber is not particularly limited so long as it neutralizesthe acid solution. Examples of the basic solution include an ethanolsolution of triethylamine, an aqueous sodium hydrogen carbonatesolution, an aqueous sodium hydroxide solution, and potassium hydroxide.The ethanol solution of triethylamine is preferable because an excessamount of alkali remaining in fibers after neutralization reaction iseasily removed.

Moreover, a method of the contacting is not particularly limited and maybe performed by spraying the basic solution to the first precursorfiber. However, the first precursor fiber is preferably allowed to passthrough a bath of the basic solution.

In particular, when the step of obtaining a first precursor fiber isperformed by the first method, and when the acid solution is thepolyphosphoric acid and the basic solution is the ethanol solution oftriethylamine, it is preferable that the first precursor fiber beallowed to pass through a bath of the ethanol solution of triethylaminefor from 5 seconds to 30 seconds.

The above-described method can effectively neutralize the acid solutionin the first precursor fiber to be removed.

Here, the precursor fiber may be washed with water or alcohol before orafter washed with the basic solution.

When the step of obtaining a first precursor fiber is performed by thesecond method, the step of obtaining a second precursor fiber isperformed as a step of contacting the first precursor fiber with asecond basic solution, and neutralizing the second acid solutionremaining in the first precursor fiber to be removed, to thereby obtaina second precursor fiber.

The second basic solution is not particularly limited so long as it canneutralize the second acid solution. Examples of the second basicsolution include an ethanol solution of triethylamine, an aqueous sodiumhydrogen carbonate solution, an aqueous sodium hydroxide solution, andpotassium hydroxide. Among them, the ethanol solution of triethylamineis preferable because excess of alkali remaining in fibers afterneutralization reaction is easily removed.

A method of the contacting is not particularly limited and may beperformed by spraying the second basic solution to the first precursorfiber. However, the first precursor fiber is preferably allowed to passthrough a bath of the second basic solution.

In particular, when the second acid solution is the methanesulfonic acidand the second basic solution is the ethanol solution of triethylamine,the first precursor fiber is preferably allowed to pass through a bathof the ethanol solution of triethylamine for from 5 seconds to 30seconds.

The above-described method can effectively neutralize the second acidsolution in the first precursor fiber to be removed.

Here, the precursor fiber may be washed with water or alcohol before orafter washed with the second basic solution.

<Step of Producing Carbon Fibers>

The step of producing carbon fibers is a step of heating the secondprecursor fiber at a temperature of from 1,000° C. to 1,600° C. in aninert gas atmosphere to turn the second precursor fiber into a carbonfiber.

When a heating temperature in the step of producing carbon fibers isfrom 1,200° C. to 1,400° C., the PBI carbon fibers that are moreexcellent in elastic modulus and strength can be produced.

Moreover, as described above, the PBI fibers have property ofmaintaining their fiber shapes even if the PBI fibers are subjected tohigh-speed carbonization treatment at a rapid temperature increasingrate.

Therefore, a temperature increasing rate in the heating is notparticularly limited and can be the following: from such a low-speedtemperature increasing rate as 5° C./min through such a high-speedtemperature increasing rate as a range of from 15° C./sec to 1,000°C./sec.

Here, the inert gas is not particularly limited. Examples of the inertgas include nitrogen and argon gas.

The method for producing the PBI carbon fiber may further include a stepof graphitizing the PBI carbon fibers. This step is performed by heatingthe PBI carbon fibers at higher temperature after the step of producingcarbon fibers or successively after the step of producing carbon fibers,in order to control mechanical properties (e.g., elastic modulus andstrength) of the PBI carbon fibers obtained through the carbonization.

A heating temperature in the graphitizing step (a heating step to beperformed successively with the carbonization step in some cases) is notparticularly limited but is preferably from 2,000° C. to 3,200° C.Setting the heating temperature in such a range makes it possible toproduce the carbon fibers having sufficient mechanical properties athigh carbonization yield and high density.

Note that, the graphitizing step is preferably performed in an inert gassimilar to the step of producing carbon fibers.

Note that, the method for producing the PBI carbon fiber may furtherinclude a surface treatment and a step of performing application ofsizing performed in known processes of producing carbon fibers.

EXAMPLES (Preparation of Precursor Fiber) <PBI Precursor Fiber 1>

First, terephthalic acid (1 mol) (available from Wako Pure ChemicalIndustries, Ltd., Distributor Code No. 208-08162) and4,4′-bephenyl-1,1′,2,2′-tetraamine (1 mol) (available from Aldrich,Distributor Code No. D12384), each of which is a raw material of apolymer, were allowed to proceed to polycondensation reaction inpolyphosphoric acid (available from Sigma-Aldrich, Distributor Code No.208213) serving as a first acid solution, to thereby prepare a reactionsolution including poly2,2′-(p-phenylene)-5,5′-bibenzimidazole as a PBIpolymer.

Next, the reaction solution was charged into a water bath serving as acoagulation bath. Then, the PBI polymer was coagulated so as to have afiber shape, to thereby obtain a first coagulated matter (step ofobtaining a first coagulated matter).

The first coagulated matter was stirred in dimethylacetamide (DMAc) towash impurities. Then, the first coagulated matter was stirred in anaqueous sodium hydrogen carbonate solution (concentration: 5 wt %) toneutralize the first acid solution in the first coagulated matter to beremoved, to thereby obtain a second coagulated matter of the PBIpolymer. Next, the second coagulated matter was washed with water andalcohol, and was dried at 240° C. under vacuum for 1 day (step ofobtaining a second coagulated matter).

Note that, it is known that polycondensation reaction of the PBI polymerproceeds in a substantially quantitative manner. It was confirmed thatwhen the step of obtaining a second coagulated matter was omitted andthe first coagulated matter was directly dried, a yield; i.e., an amountrelative to a theoretically determined amount of the PBI polymer in thefirst coagulated matter was 110% or more, which means that the firstacid solution (polyphosphoric acid) remained in the PBI polymer. Whenthe step of obtaining a second coagulated matter was performed, theyield was about 98%.

Next, the second coagulated matter was dissolved in methanesulfonic acid(available from Wako Pure Chemical Industries, Ltd., Distributor CodeNo. 138-01576) serving as a second acid solution, to thereby prepare araw liquid for spinning, the raw liquid including the second coagulatedmatter in an amount of 3.2 wt %.

By wet-type spinning, the raw liquid for spinning was charged into awater bath serving as a coagulation bath, and was allowed to passthrough a multi-hole nozzle member including 402 nozzle holes, tothereby eject a fiber bundle of 402 fibers. The fiber bundle was woundby a winding device, to thereby obtain a first precursor fiber of thePBI polymer (step of obtaining a first precursor fiber). Here, thewet-type spinning was performed under application of tension so that ajet stretch ratio represented by winding speed/discharge linear velocitywas 1.5. Moreover, a diameter of each of the nozzle holes of themulti-hole nozzle member was set so that a diameter of one firstprecursor fiber constituting the fiber bundle was 20 μm.

Next, the first precursor fiber was allowed to pass through a bath of anethanol solution of triethylamine serving as a second basic solution for30 seconds. Then, the second acid solution in the first precursor fiberwas neutralized to be removed, to thereby obtain a second precursorfiber of the PBI polymer. After that, the second precursor fiber waswashed with water and was dried (step of obtaining a second precursorfiber).

In order to confirm whether the second acid solution in the obtainedsecond precursor fiber remains or not, CHNS elemental analysis wasperformed. Here, the CHNS elemental analysis is performed by detecting asulfur component (S component) in the methanesulfonic acid serving asthe second acid solution.

As a result of the analysis, it was confirmed that the sulfur component(S component) was not detected in the second precursor fiber and themethanesulfonic acid was completely neutralized to be removed.

As described above, PBI precursor fiber 1 serving as the secondprecursor fiber was prepared.

<PBI Precursor Fiber 2>

PBI precursor fiber 2 was prepared in the same manner as in the methodfor preparing the PBI precursor fiber 1 except that a diameter of eachof the nozzle holes of the multi-hole nozzle member was changed foradjustment so that a diameter of one fiber was 11 μm.

<PBI Precursor Fiber 3>

In the preparation of the PBI precursor fiber 1, the step of obtaining asecond precursor fiber was omitted and was replaced with the followingprocedures. Specifically, the first precursor fiber was allowed to passthrough a bath of water for 30 seconds. Then, the first precursor fiberwas washed with water and was dried, to thereby obtain the secondprecursor fiber.

PBI precursor fiber 3 was prepared in the same manner as in the methodfor preparing the PBI precursor fiber 1 so that a diameter of one fiberwas 11 μm.

When the PBI precursor fiber 3 was subjected to the CHNS analysis, itwas confirmed that about 8% of the sulfur component (S component) wasdetected in the second precursor fiber and the methanesulfonic acid wasnot completely neutralized to be removed.

<PBI Precursor Fiber 4>

PBI precursor fiber 4 was prepared in the same manner as in thepreparation of the PBI precursor fiber 1 except that some procedureswere changed in the following manners. Specifically, the multi-holenozzle member was replaced with a single hole nozzle member having adiameter of a nozzle hole was 250 μm. A fiber was obtained so that onefiber was adjusted to have a fiber diameter of 40 μm. The fiber was notsubjected to the step of obtaining a second precursor fiber and wasdirectly dried, to thereby obtain PBI precursor fiber 4.

(Carbonization of Precursor Fiber)

Example 1

The PBI precursor fiber 1 serving as the second precursor fiber washeated from room temperature to a predetermined heating temperature of1,000° C. at a temperature increasing rate of 10° C./min in a nitrogenatmosphere. Moreover, the PBI precursor fiber 1 was continued in heatingfor 10 minutes at the predetermined heating temperature and was turnedinto carbon fibers, to thereby produce PBI carbon fibers according toExample 1 (step of producing carbon fibers). Here, the PBI carbon fibersaccording to Example 1 each had a diameter of 16 μm. Moreover, the PBIcarbon fibers according to Examples 2 to 7, which will be describedhereinafter, each had the same diameter as the above.

Example 2

PBI carbon fibers according to Example 2 were produced in the samemanner as in the step of producing carbon fibers of Example 1 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,100° C.

Example 3

PBI carbon fibers according to Example 3 were produced in the samemanner as in the step of producing carbon fibers of Example 1 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,200° C.

Example 4

PBI carbon fibers according to Example 4 were produced in the samemanner as in the step of producing carbon fibers of Example 1 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,300° C.

Example 5

PBI carbon fibers according to Example 5 were produced in the samemanner as in the step of producing carbon fibers of Example 1 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,400° C.

Example 6

PBI carbon fibers according to Example 6 were produced in the samemanner as in the step of producing carbon fibers of Example 1 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,500° C.

Example 7

PBI carbon fibers according to Example 7 were produced in the samemanner as in the step of producing carbon fibers of Example 1 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,600° C.

Example 8

The PBI precursor fiber 2 serving as the second precursor fiber washeated from room temperature to a predetermined heating temperature of1,000° C. at a temperature increasing rate of 10° C./min in a nitrogenatmosphere. Moreover, the PBI precursor fiber 2 was continued in heatingfor 10 minutes at the predetermined heating temperature and was turnedinto carbon fibers, to thereby produce PBI carbon fibers according toExample 8 (step of producing carbon fibers). Here, the PBI carbon fibersaccording to Example 8 each had a diameter of 9 μm. Moreover, the PBIcarbon fibers according to Examples 9 to 14, which will be describedhereinafter, each had the same diameter as the above.

Example 9

PBI carbon fibers according to Example 9 were produced in the samemanner as in the step of producing carbon fibers of Example 8 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,100° C.

Example 10

PBI carbon fibers according to Example 10 were produced in the samemanner as in the step of producing carbon fibers of Example 8 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,200° C.

Example 11

PBI carbon fibers according to Example 11 were produced in the samemanner as in the step of producing carbon fibers of Example 8 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,300° C.

Example 12

PBI carbon fibers according to Example 12 were produced in the samemanner as in the step of producing carbon fibers of Example 8 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,400° C.

Example 13

PBI carbon fibers according to Example 13 were produced in the samemanner as in the step of producing carbon fibers of Example 8 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,500° C.

Example 14

PBI carbon fibers according to Example 14 were produced in the samemanner as in the step of producing carbon fibers of Example 8 exceptthat the predetermined heating temperature was changed from 1,000° C. to1,600° C.

Comparative Example 1

PBI carbon fibers according to Comparative Example 1 were produced inthe same manner as in the step of producing carbon fibers of Example 1except that the PBI precursor fiber 1 was changed to the PBI precursorfiber 3 and the PBI precursor fiber 3 was turned into carbon fibers.

Comparative Example 2

PBI carbon fibers according to Comparative Example 2 were produced inthe same manner as in the step of producing carbon fibers of Example 6except that the PBI precursor fiber 1 was changed to the PBI precursorfiber 4 and the PBI precursor fiber 4 was turned into carbon fibers.

<Confirmation of Structure Using Electron Microscope>

FIGS. 1A to 1D are images (SEM images) presenting cross sections of thePBI carbon fibers according to Example 3, Example 10, and ComparativeExample 1, which are obtained through an electron microscope. Here, FIG.1A is an image presenting cross sections of the PBI carbon fibersaccording to Example 3 obtained through an electron microscope; FIG. 1Bis an image presenting cross sections of the PBI carbon fibers accordingto Example 10 obtained through an electron microscope; and FIGS. 1C and1D are images presenting cross sections of the PBI carbon fibersaccording to Comparative Example 1, which are obtained through anelectron microscope.

As presented in FIGS. 1A to 1D, it is confirmed that the PBI carbonfibers according to Examples 3 and 10 each have a cross-sectional shapeof nearly perfect circle and are carbon fibers each of which is hardlyadhered to another fiber. Meanwhile, it is confirmed that the PBI carbonfibers according to Comparative Example 1 have a cross-sectional shapeof ellipse and are carbon fibers each of which is strongly adhered toanother fiber.

<Single Fiber Tensile Test>

One fiber of each of the PBI carbon fibers according to Examples 1 to 14was subjected to a single fiber tensile test according to the JIS7606method to measure the fiber for elastic modulus in tension and tensilestrength.

Measurement results are presented in FIGS. 2A and 2B. Here, FIG. 2A is agraph presenting measurement results of elastic modulus in tension, andFIG. 2B is a graph presenting measurement results of tensile strength.Each value in FIGS. 2A and 2B is presented by a histogram and is anaverage value determined from values of the tests performed 10 times.The error bars present both maximum values and minimum values during thetest.

As presented in these FIGS. 2A and 2B, it is confirmed that all of thePBI carbon fibers according to Examples 1 to 14 have an elastic modulusin tension of 100 GPa or more, which is a high value, and have anelastic modulus in tension of 150 GPa or more, which is a higher value.Moreover, it is confirmed that all of the PBI carbon fibers have anelastic modulus in tension of 0.8 GPa or more, which is a high value. Itis believed that the PBI carbon fibers of the present invention canachieve the above-described high values of elastic modulus in tensionand tensile strength even if each of the PBI carbon fibers has a largediameter (e.g., 9 μm and 16 μm), which is advantageous. Among them, itis confirmed that the PBI carbon fibers according to Examples 3 to 5 and10 to 12, which were obtained at a carbonization treatment temperatureof from 1,200° C. to 1,400° C., could achieve relatively high elasticmodulus in tension and relatively high tensile strength.

Note that, a single fiber could not be extracted from the PBI carbonfibers according to Comparative Example 1 because each fiber wasstrongly adhered to another fiber. Therefore, the PBI carbon fibersaccording to Comparative Example 1 could not be measured for elasticmodulus in tension and tensile strength. It is believed that theobtained carbon fibers have low elastic modulus in tension and lowtensile strength. This is because when solvent molecules remainingthereon as salts are released through a thermal treatment, some defectsare generated in the carbon fibers, and the above-described defectsserve as a starting point of breakage in the carbon fibers.

Moreover, when the PBI carbon fibers according to Comparative Example 2were subjected to the single fiber tensile test, the PBI carbon fibershad an elastic modulus in tension of 85 GPa and a tensile strength of720 MPa.

<Presumption of Reachable Strength>

The PBI carbon fibers according to Example 11 (a carbonization treatmenttemperature is 1,300° C.), which are particularly excellent in bothelastic modulus in tension and tensile strength, were used to performedpresumption of reachable strength based on the following ReferentialDocument 1. FIG. 3 is an explanatory view presenting presumptionconditions of the reachable strength. Here, the reachable strength meansa defect-free strength presumed in the following manner in considerationof a notch tip portion at which stress is concentrated. Specifically, aspresented in FIG. 3, a surface notch is introduced into carbon fibersthrough focused ion beams. The obtained carbon fibers are subjected tothe aforementioned single fiber tensile test, to thereby obtain thereachable strength. This reachable strength can be calculated by thefollowing Mathematical Formulas (1) and (2).

$\begin{matrix}{\sigma_{N} = {\frac{1}{\alpha}\sigma_{0}}} & (1) \\{\alpha = {1 + {2\sqrt{\frac{c}{\rho}}}}} & (2)\end{matrix}$

Here, in the Mathematical Formulas (1) and (2), σ₀ represents reachablestrength, σ_(N) represents a value obtained by dividing a tensile loadby a cross-sectional area of the fiber, α represents a percentage ofstress concentration, c represents a notch depth, and ρ represents aradius of curvature of a notch tip portion.

When the PBI carbon fibers according to Example 11 were measured for theabove reachable strength, it was confirmed that a presumption value ofits reachable strength was 5.2 GPa, which was a considerably high value.This is because defects in the fibers can be reduced by optimizing theconditions of the step of obtaining a first precursor fiber and the stepof obtaining a second precursor fiber. Thereby, it can be expected toachieve a value of tensile strength that is larger than the valuesobtained in the Examples.

Referential Document 1; M. Shioya, H. Inoue, Y. Sugimoto, Carbon, v65,63-70 (2013)

(Rapid Carbonization)

Example 15

The PBI precursor fiber 1 serving as the second precursor fiber wassubjected to rapid carbonization in the following manner. Specifically,the PBI precursor fiber 1 was rapidly heated from room temperature to1,040° C. for 0.2 seconds in a nitrogen atmosphere using Curie PointPyrolyzer (available from Japan Analytical Industry Co., Ltd.) and wasretained for 5 seconds. Thereby, PBI carbon fibers according to Example15 were produced.

Example 16

PBI carbon fibers according to Example 16 were produced in the samemanner as in the method for producing the PBI carbon fibers according toExample 15 except that the PBI precursor fiber 1 was changed to the PBIprecursor fiber 2.

<Confirmation of Structure Using Electron Microscope>

FIGS. 4A to 4B are images (SEM images) presenting cross sections of thePBI carbon fibers according to Examples 15 and 16, which are obtainedthrough an electron microscope. Here, FIG. 4A is an image presentingcross sections of the PBI carbon fibers according to Example 15 obtainedthrough an electron microscope, and FIG. 4B is an image presenting crosssections of the PBI carbon fibers according to Example 16 obtainedthrough an electron microscope.

As presented in FIGS. 4A and 4B, the PBI carbon fibers according toExamples 15 and 16 each have a cross-sectional shape of nearly perfectcircle and are carbon fibers each of which is hardly adhered to anotherfiber.

(Properties of PBI Carbon Fibers)

The PBI carbon fibers of the present invention were measured fordensity, crystallinity, and microvoids (pores) in order to verify thatproperties of the PBI carbon fibers were different from properties ofother carbon fibers.

<Measurement of Density>

The PBI carbon fibers according to Examples 1 to 6 and 8 to 13 were eachmeasured for density by a sink-float method. Measurement results of theobtained densities are presented in FIG. 5.

As presented in this FIG. 5, among the densities of the PBI carbonfibers according to Examples 1 to 6 and 8 to 13, the highest density wasabout 1.7 g/cm³ at most.

It is found that each of the PBI carbon fibers according to the presentinvention has a density lower than densities of other carbon fibersbecause densities of commercially available products of PAN carbonfibers are within a range of from 1.75 g/cm³ to 1.85 g/cm³.

<Measurement of Crystallinity>

First, plane interval c/2 of carbon network planes and stack thicknessL_(c) of carbon network planes were measured as parameters indicatinggraphite crystallinity of carbon fibers. FIG. 6A is a schematic viewpresenting plane interval c/2 of carbon network planes and stackthickness L_(c) of carbon network planes in a graphite crystal. Notethat, reference signs 1 a, 1 b and 1 c in FIG. 6A represent carbonnetwork planes.

The measurement of the plane interval c/2 of the carbon network planesand the stack thickness L_(c) of the carbon network planes was performedby measuring a wide angle X-ray diffraction profile with an X-raydiffraction device using CuKα rays monochromatized with a Ni filter asan X-ray source. Specifically, in the optical system for an equatorialdirection illustrated in FIG. 6B, the plane interval c/2 of carbonnetwork planes and the stack thickness L_(c) of carbon network planeswere obtained from the peak of (002) observed at 2θ=26° in theequatorial direction profile. Note that, FIG. 6B is a schematic viewindicating an optical system in measuring a wide angle X-ray diffractionprofile, where the equatorial direction is a direction in which thedetector is perpendicular to the fiber axis.

The plane intervals c/2 and the stack thicknesses Lc of the PBI carbonfibers according to Examples 6 and 13 (carbonization treatmenttemperature of 1,500° C.) are presented in Table 1 described below.

Moreover, the plane intervals c/2 and the stack thicknesses Lc of thePBI carbon fibers according to Examples 6 and 13, which were obtainedthrough a graphitization treatment under heating at a graphitizationtemperature of 2,800° C., are also presented in Table 1 described below.

TABLE 1 Carbonization • graphitization temperature (° C.) c/2 (nm) Lc(nm) Example 13 (9 μm) 1,500 0.355 1.47 Example 6 (16 μm) 1,500 0.3521.56 Example 13 (9 μm) 2,800 0.342 9.41 Example 6 (16 μm) 2,800 0.3419.43

The plane intervals c/2 and the stack thicknesses Lc of the PBI carbonfibers according to Examples 6 and 13 described in Table 1 weresubstantially the same values as the plane intervals c/2 and the stackthicknesses Lc of PAN carbon fibers subjected to almost the samecarbonization treatment (carbonization treatment of 1,500° C.) as theabove, which are described in the below-described Referential Document 2and Referential Document 3. However, the PBI carbon fibers of thepresent invention can be distinguished from the pitch carbon fibersbecause the PBI carbon fibers have wider plane intervals c/2 and smallerstack thicknesses Lc than those of pitch carbon fibers subjected toalmost the same carbonization treatment as the above. That is, the PBIcarbon fibers according to the present invention have wider planeintervals c/2 and smaller stack thicknesses Lc than those of the pitchcarbon fibers.

Moreover, as presented in FIG. 7, the plane intervals c/2 and the stackthicknesses Lc of the PBI carbon fibers according to Examples 6 and 13,which were subjected to a graphitization treatment at 2,800° C., havenarrower stack thicknesses Lc compared to PAN graphite fibers and pitchgraphite fibers, which were subjected to almost the same graphitizationtreatment as described in the below-described Referential Document 2 andReferential Document 3. Therefore, the PBI carbon fibers can bedistinguished from the PAN carbon fibers and the pitch carbon fibers.

Referential Document 2; E. Fitzer, Carbon 27, 5, 621 (1989)

Referential Document 3; A. Takaku, et al., J. Mater. Sci., 25, 4873(1990)

<Measurement of Microvoids>

As parameters evaluating the carbon fibers for microvoids (pores),volumes and average cross-sectional areas of microvoids in the carbonfibers were measured. The measurement of the volumes and the averagecross-sectional areas of the microvoids in the carbon fibers wasperformed by measuring a small angle X-ray diffraction profile with anX-ray diffraction device using CuKα rays monochromatized with a Nifilter as an X-ray source. Specifically, in the optical system for anequatorial direction illustrated in FIG. 6B, the volumes and the averagecross-sectional areas of the microvoids were determined from thescattering patterns observed in the equatorial direction profile of arange of 2θ=0.5° through 8°. Here, the analysis method and thecalculation method were performed according to the methods described inthe Referential Document 3.

Regarding the volume and the average cross-sectional area of themicrovoids, T300 (available from Toray Industries, Inc.) (ReferentialExample 1) and IMS 60 (available from Toho Tenax Co., Ltd.) (ReferentialExample 2) as commercially available products of typical PAN carbonfibers were used for comparison.

First, volume percentages of the microvoids of the PBI carbon fibersaccording to Examples 1 to 6 and 8 to 13 are presented in FIG. 8. Aspresented in FIG. 8, compared to values of the Referential Example 1 andthe Referential Example 2 (Referential Example 1: 4.9%, ReferentialExample 2: 5.7%), the volumes of the microvoids of the PBI carbon fibersaccording to Examples 1 to 6 and 8 to 13 are similar to the above valuesor are lower than the above values, which indicates that occurrence ofmicrovoids causing breakage is low.

Next, average cross-sectional areas of the microvoids of the PBI carbonfibers according to Examples 1 to 6 and 8 to 13 are presented in FIG. 9.As presented in FIG. 9, a considerable difference among the averagecross-sectional areas of the microvoids of the PBI carbon fibersaccording to Examples 1 to 6 and 8 to 13 cannot be found. However, thevalues of the average cross-sectional areas of the microvoids presentedin FIG. 9 are considerably low; i.e., about half the values ofReferential Example 1 and Referential Example 2 (Referential Example 1:2.52 nm², Referential Example 2: 2.11 nm²).

REFERENCE SIGNS LIST

-   -   1 a, 1 b, 1 c Carbon network planes    -   c/2 Plane interval of carbon network planes    -   L_(c) Stack thickness of carbon network planes

1. A polybenzimidazole carbon fiber comprising: a structure obtained byturning a precursor fiber including polybenzimidazole into a carbonfiber under application of heat, wherein the polybenzimidazole includesa structure represented by General Formula (1) or General Formula (2)below as a structural unit, and wherein the polybenzimidazole carbonfiber has an elastic modulus in tension of 100 GPa or more and a tensilestrength of 0.8 GPa or more:

where in the General Formulas (1) and (2), R¹ and R³ each represent atrivalent or tetravalent group of one selected from the group consistingof aryl groups and unsaturated heterocyclic groups that are expressed byany one of Structural Formulas (1) to (10) below, and R² represents abivalent group of one selected from the group consisting of aryl groupsand unsaturated heterocyclic groups that are expressed by any one of theStructural Formulas (1) to (10), alkenylene groups including from 2 to 4carbon atoms, an oxygen atom, a sulfur atom, and a sulphonyl group:


2. The polybenzimidazole carbon fiber according to claim 1, wherein thepolybenzimidazole carbon fiber is a continuous fiber having a fiberdiameter of 8 μm or more.
 3. A method for producing a polybenzimidazolecarbon fiber, the method comprising: spinning, in an acid solution, apolymer including polybenzimidazole including a structure represented byGeneral Formula (1) or General Formula (2) below as a structural unit,to thereby obtain a first precursor fiber of the polymer; contacting thefirst precursor fiber with a basic solution, and neutralizing the acidsolution remaining in the first precursor fiber to be removed, tothereby obtain a second precursor fiber; and heating the secondprecursor fiber at a temperature of from 1,000° C. to 1,600° C. under aninert gas, to thereby turn the second precursor fiber into a carbonfiber:

where in the General Formulas (1) and (2), R¹ and R³ each represent atrivalent or tetravalent group of one selected from the group consistingof aryl groups and unsaturated heterocyclic groups that are expressed byany one of Structural Formulas (1) to (10) below, and R² represents abivalent group of one selected from the group consisting of aryl groupsand unsaturated heterocyclic groups that are expressed by any one of theStructural Formulas (1) to (10), alkenylene groups including from 2 to 4carbon atoms, an oxygen atom, a sulfur atom, and a sulphonyl group:


4. The method for producing a polybenzimidazole carbon fiber accordingto claim 3, wherein the acid solution is polyphosphoric acid and thebasic solution is an ethanol solution of triethylamine.
 5. The methodfor producing a polybenzimidazole carbon fiber according to claim 4,wherein the contacting is allowing the first precursor fiber to passthrough a bath of the ethanol solution of triethylamine for from 5seconds to 30 seconds to neutralize the polyphosphoric acid remaining inthe first precursor fiber to be removed.
 6. The method for producing apolybenzimidazole carbon fiber according to claim 3, wherein thespinning further comprises: coagulating, in a coagulation bath, areaction solution of the polymer obtained through polymerization in afirst acid solution, to thereby obtain a first coagulated matter of thepolymer; contacting the first coagulated matter with a first basicsolution to neutralize the first acid solution remaining in the firstcoagulated matter to be removed, to thereby obtain a second coagulatedmatter; and dissolves the second coagulated matter in a second acidsolution to prepare a raw liquid for spinning, and spins the raw liquidfor spinning, to thereby obtain a first precursor fiber of the polymer,and wherein the contacting is contacting the first precursor fiber witha second basic solution, and neutralizing the second acid solutionremaining in the first precursor fiber to be removed, to thereby obtaina second precursor fiber.
 7. The method for producing apolybenzimidazole carbon fiber according to claim 6, wherein the firstacid solution is polyphosphoric acid, the first basic solution is anaqueous sodium hydrogen carbonate solution, the second acid solution ismethanesulfonic acid, and the second basic solution is an ethanolsolution of triethylamine.
 8. The method for producing apolybenzimidazole carbon fiber according to claim 7, wherein thecontacting is allowing the first precursor fiber to pass through a bathof the ethanol solution of triethylamine for from 5 seconds to 30seconds, and neutralizing the methanesulfonic acid remaining in thefirst precursor fiber to be removed.
 9. The method for producing apolybenzimidazole carbon fiber according to claim 3, wherein a heatingtemperature of the heating is a temperature of from 1,200° C. to 1,400°C.